One of two Wisconsin girls who attempted to kill a classmate to win favor with a fictional horror character named Slender Man will find out Thursday how long she will spend in a mental hospital.

Anissa Weier, 16, pleaded guilty in August to being a party to attempted second-degree intentional homicide, but she claimed she wasn’t responsible for her actions because she was mentally ill. In September, a jury agreed.

She’s due back in Waukesha County Circuit Court on Thursday afternoon, where Judge Michael Bohren is expected to sentence her to at least three years in a psychiatric institution.

Weier and Morgan Geyser lured Payton Leutner into a wooded park in Waukesha, a Milwaukee suburb, in 2014. Geyser stabbed Leutner 19 times while Weier urged her on, according to investigators. Leutner survived after she crawled out of the woods to a path where a passing bicyclist found her.

Both Weier and Geyser told detectives they felt they had to kill Leutner to become Slender Man’s “proxies,” or servants, and protect their families from him. All three girls were 12 years old at the time.

Leutner’s mother, Stacie Leutner, sent the judge a letter this week in which she wrote that the trauma of the attack “has defined our lives” and that her daughter still fears for her life. For months, Payton slept with scissors under her pillow for protection, and she still keeps her bedroom windows closed and locked.

“She will struggle with the events of that day and physical and emotional scars it left for the rest of her life,” her mother wrote.

In her letter, Leutner did not ask the judge to sentence Weier to a specific length of time in the mental hospital, but she said her daughter wouldn’t feel safe if either of her attackers is released back into the community unsupervised.

“Payton has a lifetime of healing ahead of her and she deserves to be allowed to heal in an environment where she feels safe,” her mother wrote.

In a deal with prosecutors, Geyser, who did the stabbing, pleaded guilty to first-degree intentional homicide with the agreement that she isn’t criminally responsible and shouldn’t go to prison. She will be sentenced in February, and prosecutors have asked that she be given at least 40 years in a mental hospital.

Slender Man started with an online post in 2009, as a mysterious specter whose image people edit into everyday scenes of children at play. He is typically depicted as a spidery figure in a black suit with a featureless white face. He was regarded by his devotees as alternately a sinister force and an avenging angel.